When the heat medium that flows through a metal tube is heated by heating the metal tube with solar heat to use the heat of the heat medium, contact between the metal tube and the atmosphere transfers the heat of the heated metal tube to the atmosphere. This hinders effective heating of the heat medium. Thus, a glass tube is provided to surround the metal tube so that an annular void is formed between the metal tube and the glass tube. This prevents transfer of heat to the atmosphere from the metal tube, which is heated by solar heat. However, the significant difference in heat expansion rate between the metal tube and the glass tube requires an absorber that absorbs the difference in heat expansion between the metal tube and the glass tube. Since the temperature of the metal tube reaches several hundred degrees, the absorber is made of metal. A bellows is typically used as the absorber. However, when a metal bellows and a glass tube are directly connected, expansion and contraction of the bellows may damage the connection portion between the glass tube and the bellows.
Conventionally, as shown in FIG. 6, a structure has been proposed in which a center metal tube 31 and a glass tube 32 are connected by a bellows 33 and a glass-metal transitional element 34 (see Patent Document 1). The bellows 33 includes an inner end 33a, which is coupled to the center metal tube 31 by a coupling element 35, and an outer end 33b, which is connected to the glass tube 32 by the glass-metal transitional element 34.